1. Field of the Invention
This invention relates to controlling the rotational speed of an axial fan with particular application to a cooling fan of the type used in automotive vehicles to move air through a heat exchanger.
2. Description of the Prior Art
A ring fan is typical of cooling fans used in automotive environments. A ring fan includes radially extending blades which are connected at the distal ends by a ring. The ring provides added stability and rigidity to the blades and improves noise performance of the fan. The fan noise is a serious problem in automobiles, especially light trucks and sport utility vehicles.
Despite the advantages of noise reduction and cooling performance of ring fans, they have not enjoyed widespread use because the burst speed of a ring fan is generally lower than the burst speed of conventional axil fans, the burst speed being that speed at which the fan will break apart, i.e., burst, in response to centrifugal forces generated by the rotation. Naturally, automobile manufacturers use fans with very high burst speeds. Because the ring adds mass to the outside diameter of the fan, which increases centrifugal forces proportionally to the radius squared, engine driven ring fans have not met the burst speed requirements of automotive manufacturers.